合金微结构的调整对铁基块体金属玻璃室温塑性的影响(英文)

报道一种新奇的现象,即随着Fe75Mo5P10C8.3B1.7块体金属玻璃的直径从1.5mm增加到2.0mm,其室温压缩塑性反而从0.5%增加到1.8%。这主要归因于随着铁基块体金属玻璃样品直径的增加,原位形成了零星的α-Fe枝晶相,这种在边缘化的块体金属玻璃中出现的异质结构是提高当前铁基块体金属玻璃室温塑性的主要原因。     

Fracture surface morphology of compressed bulk metallic glass-matrix-composites and bulk metallic glass

The fracture morphology of Zr-based bulk metallic glass-matrix-composites (BMGCs) and Cu-based bulk metallic glass (BMG) after compression testing has been studied. The quasi-static compression fracture surface displays a mixture of three different distinct patterns: vein-like, smooth featureless and river-like features. The last one corresponds to the morphology known from tensile tests of BMGs. Moreover, randomly distributed transversal steps on the fracture plane are also present. This is in contrast to previous studies where a characteristic vein-like pattern is considered a unique feature of the fracture of BMGs under quasi-static uniaxial compression. The presence of different fracture features indicates that the development of the fracture plane occurs in a stepwise mode.     

A Ni-free Zr-based bulk metallic glass with remarkable plasticity

The effect of Nb and Pd combination on the glass forming ability (GFA) and mechanical properties of Zr₅₃Cu₃₀Nb_xPd_9?xAl₈ (x = 3.5–6.0) bulk metallic glasses (BMGs) were systematically investigated by X-ray diffractometry (XRD), differential scanning calorimetry (DSC), transmission electron microscopy (TEM), and compression test. TEM observation revealed that a nanocrystalline phase embeds in the amorphous matrix of the as-cast Zr₅₃Cu₃₀Nb_4.5Pd_4.5Al₈ alloy. A tiny nano-crystalline phase (with size about 5–20 nm) embedded uniformly in the amorphous matrix of the Zr₅₃Cu₃₀Nb_4.5Pd_4.5Al₈ alloy was observed and identified to be the tetragonal structured NbPd₃ phase based on the analyses of nano beam electron diffraction. According to the results of thermal analyses, the composition of Zr₅₃Cu₃₀Nb₅Pd₄Al₈ and Zr₅₃Cu₃₀Nb_4.5Pd_4.5Al₈ present the optimum GFA as well as thermal stability in the Zr₅₃Cu₃₀Nb_xPd_9?xAl₈ (x = 3.5–6.0) alloy system. In addition, the result of compression test shows that the yield strength significantly increases from 1700 MPa (Zr₅₃Cu₃₀Nb₅Pd₄Al₈) to 1900 MPa (Zr₅₃Cu₃₀Nb_4.5Pd_4.5Al₈). A remarkable compression plastic strain (11.2%) occurs at Zr₅₃Cu₃₀Nb_4.5Pd_4.5Al₈ BMG rod with 2 mm in diameter. This significant increase in plasticity is presumably due to the restriction on shear banding by the nano-size second phase.     

Compressive plasticity and toughness of a Ti-based bulk metallic glass

The effects of changes in sample dimensions on the compressive plasticity and fracture toughness/energy have been determined for Ti₄₀Zr₂₅Cu₁₂Ni₃Be₂₀ bulk metallic glass (BMG). Changes in sample dimensions alone produced widely different values for compressive plasticity. While these were correlated with changes to the measured Poisson’s ratio for samples of the same composition cast into different sample sizes, significant effects of testing conditions apart from the Poisson’s ratio on the measured compressive plasticity are also demonstrated. Fracture toughness testing on the same material/size that exhibited zero compressive plasticity produced both notch and fatigue pre-cracked toughness in excess of 100 MPa m^1/2, while a size effect on the magnitude of toughness was similarly demonstrated. Discussions on the source(s) of the size effect on toughness are provided in addition to demonstrating that exceptional toughness can be obtained in this BMG which exhibits essentially zero compressive plasticity under certain test conditions. The apparent critical Poisson’s ratio for plasticity/toughness is thus different in these very different types of tests.     

Understanding the mechanism for the embrittlement of a monolithic Zr-based bulk metallic glass by oxygen

Monolithic bulk metallic glasses (BMGs) with a nominal composition of Zr₆₅Cu_27.5Al_7.5 and the oxygen concentrations of 0.06 at.% and 0.68 at.% were prepared respectively. Oxygen effects on the deformation behavior and as-cast structural state were investigated. Although no crystalline phases were found in the BMG with higher oxygen concentration, the compressive plasticity was dramatically decreased. It was found that there is no direct correlation between the notable embrittlement and the initial free-volume content in the BMG. Geometrically-constrained compression tests for the BMGs shown that the processes of free-volume accumulation and shear band propagation during plastic deformation were obviously retarded by oxygen. It appears that the mechanism for the oxygen-induced embrittlement was closely related with the decreased atomic mobility by oxygen. As an indicator of atomic mobility, activation energy may be used as a parameter to evaluate the plasticity of monolithic Zr-based BMGs with different dissolved oxygen concentrations.     

Plastic deformability of metallic glass by artificial macroscopic notches

This paper reports that the plasticity of Zr-based metallic glass can be improved by creating two symmetrical semi-circular notches. Unlike the experimental findings of the samples without notches, a steady shear deformation can be created by the large-scale stress gradient around the two symmetrical notches and the plasticity of metallic glass can be enhanced to a high value of ～10% under compression tests. The improved plasticity may be due to the easy initiation of shear bands around the notches, and the consequent blocking effect of notches on the propagation of shear bands, similar to the dislocation mechanism in crystalline materials. To reveal the particular plastic deformation behavior of metallic glass, Ti₃SiC₂ ceramic and high-strength steel specimens with two symmetrical semi-circular notches were also conducted under compressive loadings; however, no enhancement in plasticity was found. It is suggested that creating a stress gradient is a particular strategy for designing metallic glasses in order to improve their plasticity.     

退火处理对Cu基块状非晶压缩性能的影响

选择400、600、743、753、873和973 K,进行30 min和3 h的等温退火,用XRD、万能试验机和SEM研究退火处理对Cu基块状非晶合金压缩性能的影响。结果表明,随着退火时间的延长,Cu基块状非晶合金的起始晶化温度逐渐降低;试样经过30 min和3 h的退火处理后,其压缩曲线均为线弹性,没有明显的非弹性屈服过程;在600、743和753 K经过30 min的退火处理后的压缩强度的变化趋势为降低-升高-降低的模式,与经过3 h退火处理的压缩强度的变化趋势相反,对压缩强度的变化原因进行了初步分析;Cu基块状非晶合金在743 K退火30 min后的压缩性能最好,压缩强度达到2138 MPa。     

Crystallization of Mg-based bulk metallic glass

1 Introduction Since INOUE et al[1,2] reported that amorphousalloy with the composition of Mg65Cu25Y10 could beproduced with thickness up to 4 mm by conventionamold casting technique, Mg-based bulk metallic glasses(BMGs) have been proposed as a new kind …     

Forming of bulk metallic glass microcomponents

The present article considers forward extrusion, closed-die forging and backward extrusion processes for fabrication of individual microcomponents from two bulk metallic glass (BMG) compositions: Mg₆₀Cu₃₀Y₁₀ and Zr₄₄Cu₄₀Ag₈Al₈. Two types of tooling were used in the present work: relatively massive die sets characteristic of cold forming operations for crystalline metals and lightweight die sets adapted to the special characteristics of BMGs. In addition to demonstrating that microcomponents of several geometries can be readily fabricated from BMGs, rheological properties are combined with crystallization kinetics to formulate a generally applicable method that can guide selection of optimal forming parameters. Finally, the use of particulate-based lubricants for BMG forming is shown to result in individual lubricant particles becoming mechanically locked into the BMG surface.     

Significant tensile ductility induced by cold rolling in Cu47.5Zr47.5Al5 bulk metallic glass

Significant tensile plasticity up to 0.7 ± 0.1% together with work-hardening and larger fracture strength was obtained in Cu_47.5Zr_47.5Al₅ bulk metallic glass (BMG) upon cold rolling with only 2.9 ± 0.3% thickness reduction. The good deformability could be attributed to the multiple pre-existing shear bands and structural inhomogeneity induced by rolling. The distributions of shear bands upon rolling can be predicted by a simplified rolling model. The underlying mechanism for the tensile plasticity was further discussed in the frame of potential energy landscape theory (PEL).     

Tribological behaviors of Zr-based bulk metallic glass versus Zr-based bulk metallic glass under relative heavy loads

As a novel engineering material, bulk metallic glass (BMG) has received much attention. However, the knowledge concerning the tribological behavior of BMG versus BMG under relatively heavy loads is still insufficient. In this study, Zr₄₁Ti₁₄Cu_12.5Ni₁₀Be_22.5 metallic glass pins and discs were prepared by copper-mold suction casting. The dry sliding friction and the wear characteristics of the as-cast Zr-based BMG versus Zr-based BMG were tested under loads of 100, 125 and 150 N, respectively, using a pin-on-disc tribological apparatus at room temperature. The worn surfaces were studied by scanning electron microscopy in order to identify the wear mechanisms. The results showed that both the coefficient of friction and the wear rate increased with both the normal load and the rotational sliding velocity. X-ray diffraction patterns recorded after the tribological experiments indicated that no sliding-induced crystallization occurred. Transmission electron microscopy was also used to confirm the amorphous of the BMGs after sliding tests. In addition, the wear mechanisms changed with the experimental conditions. For a normal load of 100 N, the main mechanisms were abrasive wear, slight grooves and micro-cracks. For higher loads, adhesive wear was predominant, accompanied by abrasive wear and deeper grooves and more micro-cracks. When the rotational sliding velocity was increased, the dominant wear changed from slight grooves to viscous flow and adhesive wear.     

Effect of pre-existing shear bands on the tensile mechanical properties of a bulk metallic glass

Bulk Zr_64.13Cu_15.75Ni_10.12Al₁₀ metallic glass has been rolled at room temperature in two different directions, and the dependences of microstructure and tensile mechanical property on the degree of deformation and rolling directions have been investigated. No deformation-induced crystallization occurs except for shear bands. Shear band formation in conjugated directions is achieved in the specimen rolled in two directions, while rolling in one direction induces shear band formation only in a single direction. Pre-existing properly spaced soft inhomogeneities can stabilize shear bands and lead to tensile plastic strain, and the efficient intersection of shear bands in conjugated directions results in work-hardening behavior, which is further confirmed by in situ tensile scanning electron microscopic observation. Based on the experimental results obtained in two different specimen geometries and finite element analysis, it is deduced that a normal-stress-modified maximum shear stress criterion rather than a shear plane criterion can describe the conditions for the formation of shear bands in uniaxial tension.     

Effects of characteristics of Mo dispersions on the plasticity of Mg-based bulk metallic glass composites

The Mg₅₈Cu_28.5Gd₁₁Ag_2.5 bulk metallic glass composites (BMGCs) dispersion strengthened by porous Mo particles with different volume fractions and particle sizes were synthesized by casting and characterized. The presence of porous Mo particles could restrict the shear band propagation. It was found for a given volume fraction of Mo particles, smaller particles would lead to more interfacial areas, shorter inter-particle free spacings, smaller confinement zone sizes than the larger particles, and consequently the improvement of compression plasticity from 10% up to 27%. Also, for a given Mo particle size, higher volume fraction would lead to larger compression plasticity. The inter-particle free spacing, as well as the confinement zone size (mean free path of shear bands), appears to be the controlling factor in limiting the propagation of shear bands which in turn affecting the plasticity of BMGCs.